This invention relates to electrical machinery such as motors and generators and more particularly to an electrical machine with an electrically commutated stator.
There are many applications which would benefit from an electric machine with reduced weight and high efficiency. Examples include electric aircraft propulsion, spacecraft mechanisms, wind turbine electricity generators, electrically propelled automobiles, etc.
Iron commonly constitutes a large portion of the weight of an electric machine. In the stator, iron is commonly used to shape the magnetic field and to transmit the torque of the device to the base of the machine. However, “coreless” electric machines do not have iron in the stator. In some cases, these coreless machines can result in an overall weight reduction due to their lack of iron.
Coreless machines must provide an alternative method for transmitting the torque of the machine to the base. The electrically conductive strands of which the stator is made do not generally have sufficient strength to transmit the torque themselves. A material such as epoxy or other adhesive is commonly used to encapsulate the stator electrical conductor strands to create a composite part with the required structural strength. The amount of encapsulant required to provide this structural strength is quite small, and excess encapsulant is detrimental both to dissipating heat out of the machine, and because it increases the weight of the machine. It is also desirable to maximize the amount of volume in the stator which is filled by the electrical conductor strands, which necessitates minimization of unnecessary encapsulant.
Coreless machines sometimes use Litz wire in the windings to reduce the eddy current losses in the conductors. Litz wire consists of many fine strands of electrically conductive material, such as copper, which are each coated with a thin layer of electrical insulation. The strands of Litz wire are generally twisted or braided to reduce skin and proximity effects at high frequency.
In 1981, Klaus Halbach published a paper which described an arrangement of magnets which has since been commonly referred to as a “Halbach array”. A Halbach array consists of several magnet segments which each have a similar or identical shape, but which have a magnetic orientation which rotates by an increment from one segment to the next adjacent segment. The result is that the magnetic field of the array is concentrated on one side of the array and cancelled on the other side of the array without the need for a ferromagnetic material such as iron to shape the field. If the magnet segments are of identical shape and the orientation increment is a fixed value, the variation of the magnetic field on the concentrated side is approximately sinusoidal.
The concentrated nature of the magnetic field of a Halbach array makes them ideally suited for use in electrical machines such as motors and generators. In rotating machines, the Halbach array can be arranged as a cylinder with the field either substantially in the radial direction or substantially in the axial direction. Furthermore, there can be a Halbach array on both sides of the winding, or there may just be a Halbach array on only one side of the winding. Having a Halbach array on each side of the winding increases the useful magnetic field in the stator winding.